For the machining of precision and quality parts, it is important to measure the machine accuracy and performance both at static conditions (low speed or stopped) and at dynamic conditions (high speed and multiple-axis). Laser interferometers have been used for the measurement of machine positioning accuracy at static conditions and relative to the position accuracy along each of the axes involved. That is, the measurements were not taken as the machine spindle was moving relative to the X, Y and Z axes involved for reasons including the fact that the laser systems which reflected a laser beam off of a retroreflector carried by the spindle could not intercept the reflected beam to make a measurement as the spindle was intentionally moved appreciably relative to these axes. The laser interferometer systems disclosed in said PCT application Ser. No. US 99/14815 do not have this severe problem.
A prior art device which can make such measurements but under relatively low speed circular spindle movement conditions uses a telescoping ball-bar (sometimes called double ball-bar or ball-bar).
The ball-bar consists of two steel balls supported by two three-point contact magnetic sockets, which are clamped to the spindle nose and on the table of the machine. The balls are connected by a telescoping bar, and movement is detected by a transducer similar to a linear variable differential transformer. The ball clamped on the table socket is the center of rotation, while the ball on the spindle socket performs circular motions.
The control system moves the spindle around a circle having the same radius as the ballbar""s length. As the path deviates from a perfect circle, the change in distance between the two ends of the device is measured by the transducer. Hence, the deviations in circular interpolation or machine geometry are detected by the telescoping ballbar. The data collected is then plotted in a polar coordinate and compared with a perfect circle.
Telescoping ballbar systems normally work with radii of 50 to 600 mm.
The invention described herein uses laser measurement systems like that disclosed in said PCT application in a manner which can measure spindle displacement error under conditions where the spindle is stopped or preferably is moved at high speeds in circular or other path configurations. In contrast to the ball-bar technique, the present invention is able presently to measure spindle displacements accurately for circular path radii varied continuously from as small as 1 mm (1/50th of the minimum ball-bar size) to 150 mm and larger at feed rates up to 4 m/sec. at a data rate up to 1000 data points per second with a file size up to 10,000 data points per run.
A 2-dimensional grid encoder model KGM 101, manufactured by Heidenhain, Traunrent, Germany, can be used to determine the tool path and sharp cornering. (A 2-dimensional grid encoder is similar to a glass scale linear encoder, except that the grid pattern used is 2-dimensional instead of 1-dimensional so that it is able to measure the displacement in two directions, that is in a plane.) However, this 2-dimensional system is very expensive and difficult to use. Furthermore, the gap between the reader head and the grid plate used therein is very small, so careful alignment is necessary to avoid a crash and damage to the glass scale.
There are many inferior prior art laser interferometer techniques for tool path measurement. For example, K. C. Lau and R. J. Hocken has developed three and five axis laser tracking systems (see U.S. Pat. No. 4,714,339, granted Dec. 22, 1987.) This patent discloses a laser interferometer using a retroreflector as target, another laser beam reflecting device and a quad-detector tracking device to point the laser beam always at the retroreflector. The position of the retroreflector is determined by the laser interferometer (radial distance) and the laser beam direction (two angles) as in a polar coordinate system. W. F. Marantette uses a machine tool position measurement employing multiple laser distance measurements; U.S. Pat. No. 5,387,969, granted Feb. 7, 1995 discloses three laser interferometers and a scanning mirror device to point the laser beam to the retroreflector. The spindle position in a plane is determined by the measurements of the two laser interferometers using the formula of triangulation. However, these systems are very complex and less accurate than the present invention.
Another inferior technique in one used by J. C. Ziegert and C. D. Mize (measurement instrument with interferometer and method) as disclosed in U.S. Pat. No. 5,428,446 granted Jun. 27, 1995. It discloses the use of a laser interferometer to replace the linear transducer in a telescoping ball-bar gauge.
Since a laser interferometer is more accurate and longer range than a linear transducer, the measurement range becomes larger. Hence, it is possible to measure non-circular tool paths, and the spindle position can be determined by trilateration. However, it cannot be used to measure small circles and it is also very expensive.
Thus, the ball-bar and other prior techniques for making dynamic tool path measurements are either inaccurate, incomplete, time consuming, or extremely costly relative to that achievable with the present invention.
Briefly, conventional laser interferometers using a retroreflector as the target mounted on the machine tool spindle can measure the linear displacement relative to a single axis thereof accurately. However, the tolerance for lateral displacement is rather small. Using preferably a single-aperture (or less desirable modified prior art two-aperture) laser target displacement measuring system like the Laser Doppler Displacement Meter (LDDM) of Optodyne, Inc. of Compton Calif. and a large flat-mirror or unusually large retroreflector as the target, the linear displacement along the laser beam direction can be measured with large tolerance on lateral displacement under small angular shifting of the target. This angular shifting problem can be minimized using a unique diverging laser beam when a flat-mirror target is used as shown in FIG. 3.
Using three such laser systems, pointing their laser head beams at 90 degree to each other, namely in the X, Y and Z axis directions, with preferably three flat-mirror targets perpendicular to the three axes respectively, the xyz-coordinate displacements of the target trajectory can be measured simultaneously. While it is preferred to place the flat mirrors on the spindle with the laser heads mounted on a stationary surface, the present invention also envisions reversing the positions of the laser heads and flat mirrors.
However, a preferred specific aspect of the present invention which reduces the equipment cost considerably to the customer is to use only one such system where the preferably stationary laser head thereof is sequentially directed along each of these axes to make three sequentially taken target displacement measurement sequences. This less costly system is only useable when the target positions can be repeated in sequence exactly, which is usually the case. In such case, three flat mirrors are preferably placed on the spindle facing in the directions of the three axes, or one flat mirror designed to be mounted sequentially on three different axially facing sides of the spindle are so placed, before each measurement sequence is taken. (As above indicated, the laser head and mirror locations can less desirably be reversed so that the laser head is mounted on the spindle target sequentially on different sides thereof and one or three mirrors are mounted on a stationary surface facing in the three directions to intercept the laser head beams.) In this form of the invention, a means is provided to synchronize the three sets of sequentially taken data so that the measurements for the corresponding positions and data sampling times of the target are related. This can be done in a number of ways, one by starting the data sampling times as the target is at the same path position when target position sampling begins. Another way is to program the tool path to do a xe2x80x9cspikexe2x80x9dmotion, for example, a rapid back and forth motion along a 45 degree direction between the axes involved.
The invention provides data collection, storage, computing, synchronizing and outputting processing means which supplies and stores information on the deviation of the actual from the programmed target (e.g. the machine tool spindle) positions for each sampled X, Y or Z axis position of the target and preferably also from the actual desired path thereof. Thus, these means have stored therein digital data representing the desired positions of the target along the various axes for the various target position sampling times involved and this desired target position data is compared with the corresponding actual measured axis target position data involved. The resulting axis position error numbers for each sampled target path position along each axis are stored for printout as path deviation numbers for each desired path position involved.
In accordance with another aspect of the invention, the data outputting means outputs the data to a printer or plotting means which displays two sets of overlaid lines, one representing the desired target path and the other the actual path in a manner where the degree of deviation is readily visible. Usually the deviations between the actual and desired target paths are so small that these differences are not visible on the overlaid lines. This problem is overcome by multiplying the axis deviation numbers by as much as a thousand or more times, adding the multiplied numbers to the desired target position numbers along the axes at the sampling times involved and then displaying on a printer or plotting device the modified actual and programmed position-indicating lines together.
If the target trajectory is to be only a circle in one plane, it is especially desirable to store the path deviation numbers for each sampling time for number printout in polar (i.e. angular) as well as axial terms. Thus, the polar deviation error for a particular target position along the circular path involved is computed from the X and Y axis position error numbers involved by computing the square root of the sums of the squares of these numbers. Also, when the target path is to be a circle in one plane, the sequentially taken data can be synchronized easily even when the X and Y axis related data taken at the same sampling rate starts with the target at different path position points. Thus a circular path is generated and displayed by simultaneously combining the data taken along the two axes, with the maximum or minimum displacement data taken along one axis corresponding in time to the mean (half-way between minimum and maximum) values of the other. The waveforms of such data is a sine-like curve representing the position data for one axis and a cosine-like curve representing the position data for the other axis when the sampling times are equally spaced periods.
The tool path R for any path configuration can be expressed as:
Ri(Xi,Yi,Zi), i=1,2, . . . Nxe2x80x83xe2x80x83(1)
Where Xi, Yi and Zi are data collected with laser system #1, #2 and #3, respectively and simultaneously where three systems are used or when one system is used sequentially as described. Once the phase relation of these three sets of data is determined as just described, the tool path Ri are determined by substituting these three sets of data into Eqn. 1. This composite tool path may be deviated from the tool path measured by 3 laser systems simultaneously if the tool path is not repeatable and the velocity of the motion is not constant. The effect of non-repeatable and non-uniform velocity of the motion can be minimized by collecting data over several cycles and using the mean values.
Similarly, for a complete 6 degree spindle motion, x, y, z, pitch, yaw, and roll, 6 laser systems can be used to measure the tool path Ri(Xi, Yi, Zi, Xxe2x80x2i, Yxe2x80x2i, Zxe2x80x2i). The pitch angle is (Xixe2x88x92Xxe2x80x2i)/d, where d is the beam separation. The yaw and roll angles are (Yixe2x88x92Yixe2x80x2)/d and (Zixe2x88x92Zxe2x80x2i)/d respectively.